I'm on holiday and away from my Beeb stuff so if anyone has a Model B and a DMM handy and could do a quick couple of measurements for me, I would be most grateful. It's just two power-off passive resistance checks on the analogue (joystick) connector, between the the two fire button inputs and the +5v line. So, as illustrated below, from I0 Pin 13 to +5v Pin 1 and from I1 Pin 10 to +5v Pin 1. It would be useful to measure each twice with the meter leads reversed so four measurements in total please. That's it and thanks!

Might be able to do this, if my multimeter probes will fit into the sockets ... Stupid newbie question, but that diagram is as if you're looking at the analogue socket from outside the case not inside, yes?

Yes, because of the 6522 having what I believe from previous testing to be an internal ~1k pull-up to 5v on the data lines. It's just something that I would check on a few Beebs if I was at home but being away, the thought is niggling me. If it is as I suspect, then the 10k's have actually reduced the overall I0/I1 pull-up resistance to ~900 ohms. No crisis, just interest and relevant to my other projects.

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Last edited by MartinB on Sat Aug 11, 2018 10:06 pm, edited 2 times in total.

DISCLAIMER: I know very little about electronics (I'm having flashbacks to A-level physics!) and I only bought my multimeter so I could check I'd recapped my PSUs properly. That being said, I *think* my approach of finding some wires, jamming one end into the socket and pressing the multimeter probe against the other end works!

As I had a DB15 to hand, I just had a quick check of my "to hand" machines and the issue 3 has the same reading as above, while the issue 7 and master both have 0.96x kOhms in both directions.
Caveat, my battery is low in my meter!

Mark - first off, I also have the excellent DT-4000 and a Fluke (the respected Model 12) and the two always read close enough for the delta not to matter.

All the Beebs look as I would expect, we’re just seeing the tolerance of the external 10k pull-ups working in parallel with the 6522 internal pull-up. The Elk doesn’t use a 6522, the joystick fire buttons are connected to discrete logic gates so we’re simply seeing the 2K2 pull-ups. The Master though does puzzle me... it’s schematic is identical to the Beeb but it’s as if the 6522 in yours doesn’t have internal pull-ups.....

Ok, thanks Mark. Maybe you’ve previously robbed the 6522 from that Master for another fix because it’s almost as if it doesn’t have one fitted!

Hah ha! You should know that pinching bits from a Master 128 is most likely too much like hard work, as in most Masters, everything except the ROMs are soldered in... (and I'm too lazy to go to that much effort when I have some Beebs to borrow bits from).

No, to the best of my knowledge, the Master 128 that I tested and give results for, is fully operational (it's also the only Master that currently has a memory/RTC/CMOS back-up battery fitted...).

So, anyway, I tested two other Masters, taking into account resistor tolerances, they all read the same - that is 10k.

So I investigated the first Master 128. Photo here of IC8:-

G65SC22P-2 in position IC8 in a Master

(G65SC22P-2 in position IC8 in a Master)

So that looks like a CMOS type. Presumably then, it does not have an internal pull-up resistance.